The system setting (or system preference for older versions of the OS) for the mouse is only visible if you have a mouse connected. If you have a Bluetooth mouse connected but the setting option still does not appear in the list in the System Settings app, you should try reconnecting it, like this...
In System Settings, choose Bluetooth. (see note, below) Find the mouse in the list under "My Devices". Hover over it. A "Disconnect" button should appear. Click it. Wait a few seconds then hover over the item again and click "Connect". The Mouse system setting should now be present.
Note: In the System Settings app it can be hard to find the item you need because Apple have chosen to group the items and put them in a "random" order. Additionally, some items are under the "General" category. Instead of trying to find an item this way, just click the View menu and choose from the alphabetic list that appears.
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